1. Field of the Invention
This invention relates to microelectronic and optoelectronic components, and more particularly, to a traveling-wave optoelectronic wavelength converter.
2. Description of the Related Art
(Note: This application incorporates a number of different references as indicated throughout the specification by numbers enclosed in brackets, e.g., [x]. A list of these different references ordered according to these numbers can be found below in the section of the specification entitled “References.” Each of these references is incorporated by reference herein.)
The present invention relates to wavelength converters of the type desirable in certain wavelength division multiplexed optical communication systems, as well as other applications where it is desirable to change the wavelength of the optical carrier of a modulated lightwave, and more particularly to optoelectronic wavelength converters in which an incoming lightwave having a first wavelength is detected by a photodetector that produces an electrical signal that in turn modulates the outgoing lightwave having a second desired wavelength.
In the prior art [1, 13, 14], lumped-element photodetectors and modulators were employed. These provide limitations on the signal bandwidth, B, and wavelength conversion efficiency, Pout/Pin, wherein Pout is the output signal power and Pin is the input signal power. The signal bandwidth is limited by the cutoff frequency, B<(2πRLCT)−1, wherein RL is the load resistance and CT is the sum of the detector and modulator capacitances. This can be severely limited because the lengths of both the photodetector and modulator need to be relatively long for efficient operation, and this results in a relatively large capacitance.
In conventional waveguide photodetectors, their optical absorption length must be relatively long to absorb all of the input light at high optical powers and provide high output photocurrent, Iph. In conventional modulators their optical interaction length, lm, must also be relatively long to provide high extinction with a relatively low applied voltage, Vm, as approximately characterized by a constant Vmlm product for a given modulation level. Because this voltage is proportional to the load resistance, Vm=IphRL, the efficiency of modulation, and thus wavelength conversion, increases in proportion to the load resistance. Thus, RL must be as large as possible for efficient wavelength conversion, but this limits the bandwidth of operation.
Therefore, there is a severe trade-off between the bandwidth of the signal and the efficiency of conversion in a lumped-element optoelectronic wavelength converter. Taking the above relationships into account, it can be shown that the wavelength conversion efficiency is limited to, Pout/Pin=KPin/B, where K is a constant of proportionality consisting of fixed geometrical factors and universal constants. Assuming reasonable parameters in an InP monolithic wavelength converter assembly [1, 12], it can be shown that B is limited to be less than about 10 Gb/s for near unity wavelength conversion efficiency. In fact, to obtain this bandwidth, the input power to the photodetector must be quite large (>50 mW), which is much larger than the saturation power of conventional photodetectors.
Thus, there is a need for a new optoelectronic wavelength converter geometry that can operate at higher bandwidths with high efficiency. There is also a need for higher saturation power photodetectors that may be compatibly integrated monolithically with the other elements of the wavelength converter in order to avoid the need for any electronic amplification. Also, to limit the required input power to the wavelength converter chip, it is desired to incorporate integrated semiconductor-optical-amplifiers (SOAs) to pre-amplify the incoming lightwave prior to entering the photodetector.
Furthermore, for these devices to be manufacturable with low cost, size, power dissipation, and weight, all of the elements of the wavelength converter must be monolithically integrable on a single semiconductor chip. This includes the widely-tunable laser needed to create the output optical lightwave at an arbitrary wavelength within the band of interest. In addition, for a variety of applications where space is at a premium, it is also desirable to be able to integrate arrays of these wavelength converters on a single semiconductor chip.